In recent years, a so-called self-luminous type display device that includes a pixel formed of a light emitting element such as a light emitting diode (LED) attracts attention. As a light emitting element used for such a self-luminous type display device, an organic light emitting diode (OLED), an organic EL element, or an electro luminescence (EL) element attracts attention and has been used for an organic EL display and the like.
Since a light emitting element such as an OLED is self-luminous type, it has the advantages of a higher visibility of a pixel than a liquid crystal display, a fast response without a need of backlight, and the like. Further, the luminance of a light emitting element can be controlled by current.
As a driving method of a display device using such a self-luminous type light emitting element, a passive matrix method and an active matrix method are known. The former has problems such as difficulty in realizing a large and high luminance display, though its simple structure. Therefore, in recent years, the active matrix method has been actively developed, in which a current flowing to a light emitting element is controlled by a thin film transistor (TFT) provided in a pixel circuit.
In the case of a display device adopting such an active matrix method, there are problems in that a current flowing to a light emitting element changes due to variations in current characteristics of driving TFTs and variations in luminance are caused.
That is, in the case of a display device adopting the active matrix method, driving TFTs for driving a current flowing to light emitting elements are used in a pixel circuit, and there are problems in that a current flowing to the light emitting elements changes due to variations in characteristics of these driving TFTs and variations in luminance are caused. Thus, suggested are various circuits for suppressing variations in luminance, in which a current flowing to light emitting elements does not change even when characteristics of driving TFTs in a pixel circuit vary.
(Patent Document 1)
Published Japanese Translation of PCT International Publication for Patent Application No. 2002-517806
(Patent Document 2)
International Publication WO 01/06484
(Patent Document 3)
Published Japanese Translation of PCT International Publication for Patent Application No. 2002-514320
(Patent Document 4)
International Publication WO 02/39420
A configuration of an active matrix display device is disclosed in Patent Documents 1 to 4. Disclosed in Patent Documents 1 to 3 is a circuit configuration in which a current flowing to light emitting elements does not change due to variations in characteristics of driving TFTs disposed in a pixel circuit. This configuration is called a current writing pixel or a current input pixel. Meanwhile, disclosed in Patent Document 4 is a circuit configuration for suppressing changes in signal current due to variations in TFTs in a source driver circuit.
FIG. 6 shows a first configuration example of an existing active matrix display device disclosed in Patent Document 1. A pixel shown in FIG. 6 comprises a source signal line 601, first to third gate signal lines 602 to 604, a current supply line 605, TFTs 606 to 609, a storage capacitor 610, an EL element 611, and an image signal inputting current source 612.
A gate electrode of the TFT 606 is connected to the first gate signal line 602, a first electrode thereof being connected to the source signal line 601 and a second electrode thereof being connected to a first electrode of the TFT 607, a first electrode of the TFT 608 and a first electrode of the TFF 609. A gate electrode of the TFT 607 is connected to the second gate signal line 603, a second electrode thereof being connected to a gate electrode of the TFT 608. A second electrode of the TFT 608 is connected to the current supply line 605. A gate electrode of the TFT 609 is connected to the third gate signal line 604, a second electrode thereof being connected to an anode of the EL element 611. The storage capacitor 610 is connected between the gate electrode of the TFT 608 and the current supply line, and holds a gate-source voltage of the TFT 608. The current supply line 605 and a cathode of the EL element 611 are inputted with respective predetermined potentials and have a potential difference therebetween.
Operations from writing of a signal current to light emission are described with reference to FIG. 7. Each component in the drawings is denoted by the same reference numeral as FIG. 6. FIGS. 7A to 7C are schematic diagrams each showing a current flow. FIG. 7D shows a relationship between currents flowing in each path in writing a signal current. FIG. 7E shows a voltage that is held in the storage capacitor 610 in writing a signal current also, namely the gate-source voltage of the TFT 608.
First, a pulse is inputted to the first gate signal line 602 and the second gate signal line 603, and the TFTs 606 and 607 are turned on. A current flowing in the source signal line at this time, namely a signal current is referred to as Idata.
Since the current Idata flows in the source signal line, a current flows in a pixel through current paths I1 and 12 as shown in FIG. 7A. The relationship between the divided currents is shown in FIG. 7D. It is needless to say that Idata=I1+I2 is satisfied.
At the moment when the TFT 606 is turned on, electric charges are not held in the storage capacitor 610 yet, thus the TFT 608 is off. Accordingly, I2 is equal to 0 whereas Idata is equal to I1. That is, during this period, current flows only in accordance with electric charges accumulated in the storage capacitor 610.
Then, electric charges are slowly accumulated in the storage capacitor 610, and thereby a potential difference begins to occur between both electrodes (FIG. 7E). When a potential difference between both electrodes being equal to Vth (FIG. 7E, point A), the TFT 608 is turned on and I2 is generated. Since Idata=I1+I2 is satisfied as described above, I1 gradually decreases, though current flows yet and electric charges are accumulated in the storage capacitor.
In the storage capacitor 610, electric charges continue to be accumulated until a potential difference between both electrodes thereof, that is, the gate-source voltage of the TFT 608 becomes equal to a desired voltage, namely a voltage (VGS) that allows the TFT 608 to supply the current Idata. When the accumulation of electric charges is completed (FIG. 7E, point B), the current I1 stops flowing, the TFT 608 supplies a current corresponding to the VGS at this time, and thereby Idata becomes equal to I2 (FIG. 7B). Thus, the steady state is reached. That is the end of the writing operation of signals. Finally, the selection of the first gate signal line 602 and the second gate signal line 603 is completed and the TFTs 606 and 607 are turned off.
Subsequently, a light emitting operation starts. A pulse is inputted to the third gate signal line 604 and the TFT 609 is turned on. Since the storage capacitor 610 holds the VGS that has been written earlier, the TFT 608 is on and the current Idata is supplied from the current supply line 605. Accordingly, the EL element 611 emits light. When the TFT 608 is set to operate in a saturation region at this time, the current Idata can flow without changes even when a source-drain of the TFT 608 voltage varies.
Such an operation that outputs a set current is called an output operation herein. The current writing pixel shown above as an example has the advantages that even when there are variations in characteristics of the TFT 608 and the like, the storage capacitor 610 holds a gate-source voltage required for flowing the current Idata, a desired current can be supplied to the EL element with accuracy, and thereby variations in luminance due to variations in characteristics of TFTs can be suppressed.
Described above is an example for correcting changes in current due to variations of driving TFTs in a pixel circuit. The same problem occurs in a source driver circuit. Disclosed in Patent Document 4 is a circuit configuration for preventing changes in signal current due to production variations of TFTs in a source driver circuit.